The steam sterilization process used to sterilize medical and hospital equipment cannot be effective unless the steam sterilant has been in contact with all surfaces of the materials being sterilized for the proper combination of time temperature, and steam quality. In porous load steam sterilizers, such as prevacuum steam sterilizers and gravity steam sterilizers, the process of sterilization is conducted in three main phases. In the first phase, air trapped within the porous materials being processed is removed. The second phase is a sterilizing stage, in which the load is subjected to steam under pressure for a recognized combination of time and temperature which are known to effect sterilization. The third phase is a drying phase in which condensation formed during the first two phases is removed by evacuating the chamber.
Air removal from the sterilization chamber may be achieved in a number of ways. For example, in a gravity steam sterilizer, the principle of gravity displacement is utilized, in which steam entering at the top gradually displaces the air through a valve in the base of the chamber. Alternatively, in a prevacuum steam sterilizer, air is removed forcibly by deep evacuation of the chamber or by a combination of evacuation and steam injection at either subatmospheric and/or superatmospheric pressures.
Any air which is not removed from the sterilizer during the air removal phase of the cycle or which leaks into the sterilizer during a subatmospheric pressure stage due to faulty gaskets, valves or seals, may form air pockets within any porous materials present. These air pockets will create a barrier to steam penetration, thereby preventing adequate sterilizing conditions being achieved for all surfaces of the materials being sterilized during the sterilizing phase. This is particularly true when porous materials such as hospital linens or fabrics are being sterilized since the air pockets prohibit the steam from reaching the interior layers of such materials. As a result, sterilization may not occur. Therefore, there is a need for an apparatus for determining the efficacy of sterilization cycles in sterilizers which operates by detecting whether there has been sufficient sterilant penetration.
One commonly used for procedure for evaluating the effectiveness of air removal during the air removal phase of a porous load sterilization cycle is known as the Bowie-Dick test. The typical Bowie-Dick test pack essentially consists of a stack of freshly laundered towels folded to a specific size. A chemical indicator sheet is then placed in the center of the pack. If the air removal within the sterilizer is insufficient, an air pocket will form in the center of the pack thereby preventing steam from contacting the steam sensitive chemical indicator sheet. The presence of the air pocket will be recorded by the failure of the indicator to undergo complete or uniform color change, indicative of adequate air removal. Although the Bowie-Dick type test is generally recognized as an adequate procedure for determining the efficacy of the air removal stage of prevacuum sterilizers, it stills presents many disadvantages. Since the test pack is not preassembled, it must be constructed every time the procedure is used to monitor sterilizer performance. The testing procedure may be somewhat inconsistent because varying factors, such as laundering, prehumidification, towel thickness and wear, and the number of towels used, alter the test results. Further, the preparation, assembly and use of the towel pack is time consuming and cumbersome. Therefore, alternative Bowie-Dick test packs have been developed to overcome these limitations.
An example of an alternative Bowie-Dick test pack is described in European Patent Application No. 90310367.9 to Hart et at. which describes a disposable test pack for steam or gas sterilizers. The Hart et at. test pack includes a container having top and bottom walls with a porous packing material disposed within the container. The packing material challenges the penetration of the sterilant by providing a restricted pathway which acts to impede the flow of the sterilant through the test pack. A removable lid seals the bottom end of the container, while a hole in the top wall of the container allows for the downward ingress of steam into the packing material within the container. The test pack includes a chemical indicator for detecting sterilant penetration. If sterilant successfully penetrates the packing material of the test pack, the chemical indicator sheet will undergo a complete color change. If the sterilant does not sufficiently penetrate the packing material, the chemical indicator will not undergo a complete uniform color change, thereby indicating inadequate air removal, or in other words, a Bowie-Dick test failure.
Parametric monitoring has been used to either monitor or control a sterilization cycle to ensure proper sterilization conditions are attained. For example, in U.S. Pat. No. 4,865,814 to Childress, an automatic sterilizer is disclosed which includes a microprocessor which monitors both the temperature and pressure levels inside the sterilization chamber and controls a heater to allow both pressure and temperature to reach predetermined levels before starting a timer. Once the timer is started, it is stopped if the pressure or temperature levels drop below a predetermined minimum. Sterilization criteria for steam sterilizers are often defined by requiring items to be sterilized to be subjected to a high quality steam at a given temperature for predetermined period of time. Since it is known that the pressure and temperature variables of saturated steam are dependent variables when saturated steam is enclosed in a sealed chamber, monitoring of these two variables can ensure that proper conditions are maintained during the sterilization cycle.
Although it is desirable to monitor environmental conditions within the sterilization chamber itself, it is even more desirable to be able to monitor the environmental conditions within or at the center of the actual load being sterilized. While external monitoring may be used, it is further desirable to have a self contained monitoring unit, which avoids having to introduce wires into the sterilization chamber, thereby potentially breaching the integrity of the chamber. In U.S. Pat. No. 3,982,893 to Joslyn discloses a system which includes a monitoring device which can be placed within a load to be sterilized. The device continuously monitors environmental conditions of the load, including at least humidity and temperature. The device generates a signal and transmits it to an antenna placed in the sterilization chamber which is wired to an outside device which controls the environmental parameters of the sterilizer. Thus, the Josyln device provides a self-contained device for controlling the operation of the sterilizer rather than testing the efficacy of the sterilization cycle by monitoring the humidity and temperature at the center of the load.
The devices used today to test the efficacy of sterilizers typically employ biological and/or chemical indicators. The Bowie-Dick test is an example of a chemical indicator test typically carried out at the start of testing each working day in order to determine the efficacy of the air removal stage of the cycle. The test is designed so detect the presence of residual air within the sterilization chamber due to leaks, failed gaskets or valves or the ingress of noncondensible gases present in the steam supply, all of which prevent adequate steam penetration into the porous mass constituting the test pack. Chemical indicator test sheets undergo a visible change from either one distinct color to another, for example, from an initial white to a final black color, upon exposure to the sterilization process. The consequence of inadequate steam penetration is a non-uniform color development across the surface of the chemical indicator test sheet. Chemical indicators, however, can be difficult to interpret, depending on the state of the color change.
Biological indicator systems provide information on the adequacy of the sterilization stage of the cycle. Biological indicator test systems employ living spores which are subjected to a sterilization cycle. After the cycle, the spores are incubated and the system detects if there is any growth. If there is no growth, it indicates that the sterilization process has been effective. Thus, biological indicators can determine whether conditions for sterilization were present, but the length of time to obtain results due to the incubation period is often at least 24 hours. Therefore, biological indicator systems are often used in conjunction with chemical indicators because the color change of the chemical indicators provides an instant result. Further, by using both chemical and biological indicators, information on both the adequacy of the air removal stage and the sterilization stage is provided.
Parametric monitoring has advantages over a chemical or biological indicator because results could be obtained instantaneously and the results can be in the form of a clear pass/fail decision. Moreover, rather than getting merely a pass/fail decision, detailed data is obtained which not only allow a pass/fail decision but also data can be obtained for allowing further analysis into the performance of the sterilizer. Therefore, what is desirable is a test pack which uses parametric measuring to determine if adequate sterilant penetration has been achieved within the test pack. More specifically, what is desirable is an alternative test pack which uses parametric measuring to determine the adequacy of the air removal stage of the sterilization cycle. What is more desirable is an alternative test pack which uses parametric measuring to not only determine the adequacy of the air removal stage but also the adequacy of the sterilization stage.